This invention relates generally to high pressure packing and deals more particularly with a packing assembly for sealing the stem and bonnet of a gate valve.
In recent years, valves have been called upon to handle increasingly higher pressures, particularly valves which are used in the oil and gas industry where the trend has been toward deeper wells in which pressures up to 30,000 psi and above are sometimes encountered. Aside from the need to handle the high pressure fluids found in such deep wells, the valves must also contend with noxious substances such as hydrogen sulfide gas which can pose serious dangers if permitted to leak. Furthermore, the valves are often exposed to high temperature fluids which can cause thermal distortion of the valve and the various components thereof, including the stem packing.
The various types of packing materials and stem packing arrangements that have been proposed in the past for critical surface valves have not proven to be entirely satisfactory in all respects. Metal packing rings have the advantage of holding up well under high pressures and temperatures. However, they do not seal effectively with the valve stem after it has been galled or otherwise damaged on its outer surface since the metal packing cannot conform adequately to surface irregularities on the stem. While polytetrafluoroethylene and similar plastic materials have proven to be highly desirable as packing materials, they are not wholly without problems when used in high pressure service. Most notably, the application of high pressure to plastic packings tends to severely extrude the packing so that it is quickly dissipated. Such materials are also highly susceptible to wear after repeated cycling of the valve stem. Moreover, plastic packings of this type are susceptible to complete destruction by fire and expand considerably at elevated temperatures short of fire conditions, resulting in a decrease in the sealing capability of the packing when high temperature fluids are handled. In these and other respects, existing packings have performed in an unsatisfactory manner in critical service, and the lack of suitable packing has contributed significantly to delays in the development of high pressure valves.